The present invention relates to the field of semiconductor processing, more specific to the field of low-κ dielectric repair after chemical mechanical polishing or plasma processing.
With advancements in ultra large-scale integration (ULSI), integrated microelectronic device dimensions are being scaled down and new challenges are defined for both front-end-of-line (FEOL) and back-end-of-line (BEOL) processing. The low-κ dielectric materials have been introduced to improve interconnects performance, more specific to reduce the resistance-capacitance (RC) delay. These materials are insulators with a dielectric constant κ lower than the κ-value for silicon dioxide (˜4.0-4.2).
Recently with the advancements towards the 45 nm technology node high porosity low-κ materials with a κ-value lower than 2.5, known as ultra low-κ (ULK) materials have been introduced as interlayer dielectric. Next to many advantages they introduce also some important challenges in semiconductor fabrication: e.g. processes like plasma etch or plasma ashing, chemical mechanical polishing (CMP) and some cleaning chemistries may affect their properties.
At first, in order to protect these ULK materials during CMP, they were usually covered by a dielectric protection layer, which was partly polished during the CMP step. Nowadays, direct CMP, the approach without a dielectric protection layer is an important route to improve performance. As a consequence the absence of a dielectric protection layer results in damage of the underlying ULK dielectric material due to its direct exposure during CMP.
For instance, there is an increased discrepancy between the κ-values of the processed ULK dielectrics and the as-deposited ones, the processed dielectrics having higher κ-values. This observation has been attributed to the presence of moisture and adsorbed chemicals (slurries, cleaning solutions, etc.) penetrating the porous network during the CMP process.
Therefore before going to the next processing step, the κ-value should be restored as much as possible to its pristine (as-deposited) value and the surface of the ULK material should be stabilized/sealed in order to prevent further moisture and/or contamination uptake.
Thermal treatments are known to recover the as-deposited κ-value of a porous dielectric, but they are not completely recovering the hydrophobicity or stabilizing the surface. As a consequence, the restored κ-value is increasing again after several days.
Silylation processes in liquid solvents with organic molecules (silylation agents) are capable to restore hydrophobicity, but these treatments end-up with κ-values still too high with respect to the pristine ones.
Treatments with mixtures of silylation agents and supercritical CO2 are known from the related field of plasma damage repair disclosed by Toma et al. in WO03/077032. They disclose treating a low-κ surface with a silylation agent in supercritical CO2 to passivate the silanol groups on the surface. Their method restores hydrophobicity, but fails to offer a solution for the ageing behavior of the processed dielectric. Degradation/ageing of the restored properties of the porous dielectric can still take place e.g. due to adsorption of moisture and/or organic contamination in the bulk. Moreover working with supercritical CO2 requires difficult experimental conditions like high temperatures and high pressures and any simplification of these experimental conditions would be of great benefit.